Demagnetization of cobalt-rare earth magnets

ABSTRACT

A cobalt-rare earth intermetallic permanent magnet having a high intrinsic coercive force is substantially or completely demagnetized by low energy means. The magnet is heated to an elevated temperature ranging from 40 to 75 percent of its Curie temperature. At such temperature it is demagnetized and the extent of this demagnetization depends largely on the specific temperature to which the magnet is heated and its selfdemagnetizing field. If necessary, an external demagnetizing field, which is at least 50 percent below the magnet&#39;&#39;s intrinsic coercive force at room temperature, is applied to the magnet at the elevated temperature to attain substantial or complete demagnetization.

Martin et a1.

[ DEMAGNETIZATION OF COBALT-RARE EARTH MAGNETS [75] Inventors: Donald L. Martin, Elnora, N.Y.;

Rollin J. Parker, Greenville, Mich.

[73] Assignee: General Electric Company,

Schenectady, NY.

22 Filed: May 25,1972

211 Appl. No.2 256,942

[52] US. Cl 148/103, l48/3l.57, 148/108 [51] Int. Cl. 1101f 1/02 [58] Field of Search 148/103, 31.57, 108, 102

[56] References Cited UNITED STATES PATENTS I 3,655,464 4/1972 Benz .l 148/103 2,295,082 9/1942 Jonas 148/103 2,837,452 6/1958 De Vos et al.... 148/103 3.039891 6/1962 Mitchell 148/103 3102002 8/1963 Wallace et al. 148/103 FOREIGN PATENTS OR APPLICATIONS 858,015 l/l96l Great Britain 148/103 I OTHER PUBLICATIONS Bozorth, R. M.; Ferromagneti sm, New York, 1951,

[111 r 3,302,935 451 Apr, 9, 1974 pp. 344, and 713 714. Buschow, K- et Permanent Mag. Materials of Rte-C0 p in Zeit. Furr Physik,"26, 1969, pp. 157-160.

Primary E.\'aniinerHyland Bizot Assistant E.\aminerW. R. Satterfield Attorney, Agent, or Firm-Jane M. Binkowski; Joseph T. Cohen; Jerome C. 'Squillaro [57] ABSTRACT specific temperature to which the magnet is heated and its self-demagnetizing'field. If necessary, an exter-, nal demagnetizing field, which is at least 50 percent below the magnets intrinsic coercive force at room temperature,'is applied to the magnet at the'elevated temperature to attain substantial or complete demagnetization.

4 Claims, N0 Drawings DEMAGNETIZATION OF COBALT-RARE EARTH MAGNETS The present invention relates generally to the art of demagnetizing cobalt-rare earth permanent magnets.

In one aspect, it relates to the use of reduced energy means to demagnetize a cobalt-rare earth permanent magnet without significantly deteriorating its magnetic properties, i.e., these properties being substantially or completely recoverable by remagnetization. I

Permanent magnets, i.e., hard" magnetic materials such as the cobalt-rare earth intermetallic compounds, are of technological importance because they can maintain a high, constant magnetic flux in the absence of an exciting magnetic field or electrical current to bring about such a field.

Within the past few years a new class of materials formaking permanent magnets has been developed, based on cobalt and rare-earth elements. The improvement over prior art materials is so great that the cobalt-rare? earth magnets stand in a class by themselves. In terms of their resistance to demagnetization the new materi- --als are from 20 to 50 times superior to conventional magnets of the Anico type, and their magnetic energy is from two to six times greater. Since the more powerful the magnet for a given size. is the smaller it can be for a given job, thecobalt-rare-earth magnets have applications forwhich prior'art materials cannot even be considered.

Demagnetization of cobalt-rare earth permanent magnets is frequently required as a part of the manufacturing process for example, shipping the magnet in the demagnetized condition is advantageous.

The high intrisiccoercive force H of the cobalt-rare earth'magnets makes it difficult to demagnetize them by normal means such as an a/c field. Demagnetization of Alnico or BaO ferrite is readily accomplished. by a coercive force H in excess of 5,000 oersteds. Specifically, when a permanent magnet material is magnetized, a magnetization value of 4111 gauss is established therein. The-shape of the magnet imposes a selfdemagnetizing field of H oersteds. Together, these useful in'the present process. Such magnets are usually sintered cobalt-rare earth intermetallic products which may be prepared by a number of techniques. Briefly, the cobalt-rare earth intermetallic material is formed and converted to particulate form. The-particles are compressed into a green body which is then sintered in a substantially inert atmosphere to produce a sintered body of the desired density.

' Cobalt-rare earth intermetallic compounds exist in a variety of phases, but the Co R intermetallic single I phase compounds (in each occurrence R designates a rare earth metal) have exhibited the bestmagnetic properties, and generally, sintered products composed of the Co R phase or containing the Co R phase in at leasta significant amount have produced thebest permanent magnets.

2,500 3,000 a/c field; however, such a field is too low for most of the cobalt-rare earth permanent magnets where the H value may exceed 15,000 oersteds at room temperature. The energy requirements for furnishing the nece'ssarydemagnetizing field for these cobalt-rare earth magnets are too high to be practical.

The-Curie temperature is the temperature above which a magnetic material loses its magnetic properties. For most cobalt-rare earth magnets, the Curie temperature generally ranges from about 500C to 735C and for some it maybe as low 'as about 375C. Thermal demagnetization by heating above the Curie temperature is inconvenient for commercial use because of the danger of permanently deteriorating the magnetic properties by this additional high temperature heat treatment. i

Briefly stated, the present process comprises providing a cobalt-rare earth intermetallic magnet having an intrinsic coercive force higher than 5,000 oersteds, heating saidmagnet to an elevated temperature ranging from 40 to 75 percent of its Curie temperature. Then, if necessary to attain substantial or complete demagnetization of the magnet, applying to'the magnet at elevated temperature a demagnetizing field which is at least 50 percent below the magnets intrinsic coercive force H at room temperature. The magnet is then perferably cooled to room temperature. The rate of cool ing of the magnet is not critical.

The present process is particularly useful for cobaltrare magnets having at room temperature an intrinsic Specifically, the present invention is particularly useful for the sintered cobalt-rare earth permanent magnets disclosed in U.S. Pat. No. 3,655,464, U.S. Pat. No. 3,655,463 and copending Ser. No. 33,224, now U.S. Pat. No. 3,695,945, all filed on Apr. 30, 1970 in the name of Mark G. Benz, and assigned to the assignee hereof, and all of which by reference are made part of the disclosure of the present application. I

Each of the aforementioned. patents and patent application discloses a process for preparing novel sintered cobalt-rare earth intermetallic products which can be magnetized to form'magnets having improved permanent magnet properties."

Briefly stateed, in U.S.Pat. No. 3,655,464 a particu-' late mixture of a base Co-R alloy and an additive Co-R alloy,"where R is a rare earth metal or metals is sintered to produce a product having a composition lying outside the Co R single phase on the rare 'earthricher side. Specifically, thebase alloy is one which at sintering temperature exists as a solid'Co R intermetallic single phase. Since the'Co R single phase may varyin composition','the base alloy may vary in compositionwhich can be determined from the phase diagram for the particular c0balt=rare earth system, or. empirically. The additive cobalt-rare earth alloy is richer in rare earth metal than the base alloy and at sintering temperature it is at least partly in liquid form and thus increases the sintering rate. The additive alloy may vary in composition and can be determined fromthe phase diagram for the particular cobalt-rare earth system orit can be determined empirically.

The base and additive alloy, in particulate form, are each used in an amount to-form a mixture which has 'a cobalt and rare earth metal content substantially correthan the base alloy is used. The procedure for forming the sintered products dissufficient to promote sintering, and generally, should be used in an amount of at least 0.5 percent by weight of'the base-additive alloy mixture. The particulate mixture is compressed into. a green body of the desired size and density. Preferably, the particles are magnetically aligned along their easy axis prior to or during compression since the greater their magnetic alignment, the better are the resulting magnetic properties.

The green body is sintered in a substantially inert atmosphere toproduce a sintered body of desired density. Preferably, the green body is sintered to produce a sintered body wherein the pores are substantially non-interconnecting, which generally is a sintered body having a density of at least about.87 percent of theoreti cal. Such non-interco'nnectivitystabilizes the permanent magnet properties of the product because the interior of the sintered product or magnet is protected against exposure to the ambient atmosphere. 7

. Sintering temperature depends largely on the particular cobalt-rare earth intermetallic material to be sintered, but it must be sufficiently high to coalesce-the component particles. Preferably, sintering is carried .out so that the pores in the'sintered product are subparticular density depends largely on the particular permanent magnet properties desired' Preferably, to

obtain a product with substantially stable permanent magnet properties, the density of the'sintered product should be one wherein the pores are substantially-noninterconnecting and this occurs usually at a density or packing ofabout 87 percent. Generally, for a number of applications,'the density of the sintered product may range from about 80 percent to 100 percent. For exampie, for low temperatureapplications, a sintered body having a density rangingdown to about 80 percent may be 'satisfact'oryp I The procedure for forming sintered products disclosed'in U.S. Pat. No..3,655,463 is substantially the same as that disclosed in U.S. Pat. No. 3,655,464 ex- 4 v ln addition, copending U.S. Pat. application Ser. No. 86,288 now U.S. Pat. No. 3,684,593 entitled Heat- 'Aged Sintered Cobalt-Rare Earth lntermetallic Product And Process, filed on Nov. 2, 1970 in the names of Mark G. Benz and Donald L. Martin and assigned to the assignee hereof, is, by reference, made part of the disclosure of the present application. Briefly stated, in U.S. Set. No.- 86,288 now U.S. Pat. No. 3,684,593

there is disclosed a process for preparing heat-aged n'ovel sintered cobalt-rare earth intermetallic products by providing a sintered cobalt-rare earth 'intermetallic product ranging in composition from a single solid Co R phase to that'compos'ed of Co R phase and a second phase of solid Co-R in anamount of up to about 30 percent by weight of the product and richer in rare earth metal content than said C0,,R, and heat-aging said product at an aging temperature within 400C below cept that an additive Co-R alloy which is solid at sinter-Q ing temperature and which is richer in rare earth metal solid intermetallic phase, generally at least about 70 percent by weight of the product, and a second solid Co-R intermetallic phase which is richer in rare earth ent in an amount of up to about 30 percent by weight of the product. Traces of other cobalt-rare earth intermetallic phases, in most instances less than one percent by weight of the product, may also be present.

' the temperature at which it was sintered to precipitate CoR phase richer in rare earth metal content than said -Co,,R in an amount sufficient to increase intrinsic and- /or normal coercive force of said product by at least l0 percent, where R is a rare earth metal or metals. Heataging is carried out in an atmosphere such as argon in which the material is substantially inert. The precipitated'Co-R phase is generally present in an amount ranging from about '1 to 15 percent by-weight of the product. The present invention is these heat-aged sintered products.

The rare earth metals useful in preparing the cobaltrare earth alloys and intermetallic compounds used in forming the sintered products are the 15 elements of the lanthanide series having atomic numbers 57 to 7i inclusive. The element yttrium (atomic number 39) is commonly included inthis group of metals and, in this specification, is considered a rare earth metal. A plurality of rare earth metals'can also be used to form the present desired cobalt-rare earth alloys or intermetallic compounds which, for example may be ternary, quartenary or which may contain an even greater number of rare earth metals as desired.

Representative of thecobalt-rare earth alloys useful in forming the sintered products are cobalt-cerium, cobalt-praseodymium, cobalt-neodymium, cobaltpromethium,cobalt samarium, cobalt-europium, cohalt-gadolinium, cobalt-terbium, cobalt-dysprosiurn,

' cobalt-holmium, cobalt-erbium, j cob'alt-thulium, co-

balt-ytterbium, cobalt-lu'teciur'n, cobalt-yttrium, cobaltlanthanum and cobalt-misch metal. Misch metal is the most common alloy of the rare earth metals which contains the metals in the approximate ratio in which they occur in their most common naturally occurring ores. Examples of specific ternary alloys includecobaltsamarium-misch metal, cobalt-cerium-praseodymium,

- metal content than the Co R phase and which is prescobalt-yttrium-praseodymium, praseodymium-misch metal. In carrying out the present process, the permanent magnet is heated to an elevated temperature which ranges from 40 to'75 percent of its Curie temperature.

7 and cobalt- The higher the temperature within this specified range,

the greater is the extent to which the magnet is demagnetized. The Curie temperature for a particular material is readily available in the art. Specifically, for the C0,,Sm intermetallic compound it is 725C, for Co Pr it is 610C, for C0,,La it is 570C, for Co Gd it is' 735C, for C0,,Mischmetal it is 500C and for Co Ce it is 375C. Heating the magnet to the elevated temperature can be carried out in air if such heating does not signifip'articularly useful for the magnet need only be brought up to the elevated temperature and the period of time which it is at temperature is not critical. 7

The extent to which the magnet is demagnetized at the present elevated temperature depends largely on the specific temperature to which it is heated and its self-demagnetizing field which is determined by its shape. Specifically, when the magnet is small and has a low effective l/d ratio, where l is length and d is the diameter, for example if it is a cylindrical bar having a low l/d ratio such as one or less than one, it has a high self-demagnetizing field. Heating such a magnet to a temperature which is 40 to 75 percent of its Curie temperature is ordinarily sufficient to substantially or completely demagnetize it. By substantial demagnetization it is meant that the open circuit induction B of the magnet has been decreased by at least about 80 percent.

As the l/d ratio of the magnet increases, its selfdemagnetizing field decreases. For magnets having a high l/d ratio, such as greater than one, the present heat treatment alone may not substantially or completely demagnetize the magnet. In such instance, to

a DC or AC field provided by means such as an AC solenoid or electromagnet or even by a permanent magnet.

Alternatively, where the cobalt-rare earth permanent magnet is substantially or completely demagnetized by heating it to a temperature only in the upper portion of the temperature ranging from 40 to '75 percent of its Curie temperature, it may be desirable to heat it to a temperature in the lower portion of this temperature range and attain substantial demagnetization by applying the said demagnetizing field to the magnet at such lower'temperature range.

As a result of the present process, cobalt-rare earth permanent magnets are demagnetized without significant deterioration of their permanent magnet properties. Specifically, the permanent magnet properties of the cobalt-rare earth magnets demagnetized in accordance with the present process are substantially or completely recoverable by remagnetization of the magnet.

All parts and percentages used herein are by' weight unless otherwise noted.

The invention is further illustrated by the following examples in which, unless otherwise noted, the conditions and procedure were as follows:

The magnetizing field was used to magnetically align along the easy axis.

All sintering was carried out in an inert atmosphere of purified argon and upon completion of the sintering, the sintered product was cooled in the same purified argon atmosphere.

The density of the sintered product is given as packing. Packing is the relative density of the material, i.e.,

it is a percent of theoretical. Packing was determined Weight Volume where 8.6g/cc. is the density of Co -,Sm.

EXAMPLE In the runs of the following table, each alloy melt was made under purified argon by induction .melting and .cast into'an ingot. The ingot was then crushed in air by means of mortar and pestle or in a jaw crusher in nitrogen and then ground in nitrogen by fluid energy milling halt-6O percent samarium, and the base and additivealloys were each used in an amount to give a composition of 63.1 percent cobalt-36.9percent samarium.

The green bodyof each run was formed by packing themixture into a rubber tube having a working space of 3/8 inch in diameter and 1 3/4 inch long. The tube. was placed in an axial magnetic field of 60,000 oersteds to align the particles along the easy axis. After aligning,

' the powder was compressed and the sample was subsequently hydrostatically pressed under 200,000 psi. The

pressed samples, i.e., green bodies, had a packing density of about percent. The green bodies were cylindrical bars. Generally, the green bodies weremachined dimensions for testp for V2 hour. In Run No. 2, the sintered product was prepared in the same manner as in Run No. 1 except that sintering temperature was 1,1 15C. The sintered products contained the Co Sm phase in an amount of about 94 percent. The resulting sintered products were then magnetized and demagnetized as shown in the following table. Y

Properties Open I Circuit Intrinsic Induction Coercive Packing length/ a Force H, Run No. diameter Permanent Magnet Treatment (gauss) (oersteds) l 95 4.45 (a)After magnetization in 60,000 oersteds at 9290 18,700

room temperature (b)Heated to 400C for 30 minutes, 8400 demagnetizing field of about -650 oersteds applied by AC field at 400C and then air cooled to room temperature.

Run No.

- Packing length/ diameter Permanent Magnet Treatment (c)After magnetization in 60,000 oersteds at room temperature, heated to 425C for minutes, demagnetizing'field of about 650 7 Open Circuit induction (g auss) oersteds applied by AC field at 425C and then air cooled to room temperature. (d)After magnetization in 60,000 oersteds at room temperature, heated to 475C for 30 minutes, demagnetizing field of about 650 oersteds applied by AC field at 475C and then air cooled to room temperature.

(e)After magnetization in 60,000 oersteds "at room temperature, heated to 525C for 30 minutes, demagnetizing field of about 650 air cooled to room temperature.

(f)After magnetization in 60,000 oersteds at room temperature.

(g)Heated to 400C for 30 minutes and air cooled to room temperature. (h)After magnetization in 60,000 oersteds at room temperature, heated to 425C for 30 minutes and air cooled to room temperature.

, (i)After magnetization in 60,000 oersteds at room temperature, heated to 475C for 30 minutes and air cooled to room temperature.

(j)After magnetization in 60,000 oersteds at room temperature.

(a)After magnetization in 60,000 oersteds at room temperature.

(b)Heate'd to 425C for 30 minutes and air cooled to room temperature.

(c)After magnetization in 60,000 oersteds at room temperature, heated to 425C for 30 minutes, demagnetizing field of about -650 oersteds applied by AC field at 525C and then Properties intrinsic Coercive Force H (oersteds) oersteds applied by AC field at 425C and then air cooled to room temperature. (d)After magnetization in 60,000 oersteds at 55 room temperature, heated to 425C for 30 minutes, demagnetizing field of about 800 oersteds applied by DC field at 425C and then air cooled to room temperature.

(e)After magnetization in 60,000 oersteds at 8870 room temperature.

(a)After magnetization in 60,000 oersteds at 8520 i 'room'temperature.

, (b)l-leated to 500C for 30 minutes and air 2280' cooled to room temperature. (c)After magnetization in 60,000 oersteds at 8260 room temperature.

(a)After magnetization in 60,000 oersteds at 5000 room temperature.

( b)Heatedto 500C for 30 minutes and air 190 A cooled to room temperature. (c)After magnetization in 60,000 oersteds at 4990 The runs in the table illustrate the present invention. Run No. l, steps (b) to (e) show that by heating to temperature and applying a relatively small demagnetizing field at temperature, the permanent magnet is substantially or completely demagnetized as illustrated by the open circuit induction which is decreased significantly or reduced to zero. Step (flshows that the open circuit induction is substantially recoverable upon remagnetization. On the other hand, Run No. 1, steps g to (i) show the extent to which certain elevated temperatures alone demagnetize the permanent magnet as illustrated by the reduction in its open circuit induction B Step (j shows that the open circuit induction is substantially recoverable, upon remagnetization of the magnet;

Run No. 2 shows the significant decrease in open circuit induction which can be attained by heating to tom- Y perature and applying a relatively small demagnetizing field in accordance with the present invention.

in Run No. 4 heating to temperature alone was sufficient to substantially demagnetize the permanent magroom temperature.

net since this magnet had a low l/d ratio, and therefore, a high self-demagnetizing field. V

What we claim as new and desire to secure by Letters Patent of the United States is:

1.-A process for treating a cobalt-rare earth 'perrn'anent magnet to demagnetize said magnet to decrease its open circuit induction B by at least percent so that said open circuit induction B is substantially or. coma vCo R phase and a second solid Co-R phase in an amount up to about 30 percent by weight of said prodnet and richer in rare earth metal content than said 3. A process for treating a cobalt-rare earth permanent magnet to demagnetize said magnet to decrease its open circuit induction B by at least 80 percent so that said open circuit induction B is substantially or completely recoverable by remagnetization of said magnet, said magnet having a length to diameter ratio greater than one and an intrinsic coercive force in excess of 5,000 oersteds and said magnet being a sintered product wherein the pores are substantially noninterconnecting and having a density ranging from about 87 to 100 percent and consisting essentially of compacted particulate alloy consisting essentially of a composition ranging from a single solid Co R phase to a Co R phase and a second solid Co-R phase in'an amount up to about 30 percent by weight of said product and richer in rare earth metal content than said Co R phase, where R is a rare earth metal or metals, which comprises bringing said magnet up to an elevated temperature ranging from 40 to 75 percent of its Curie temperature in an atmosphere in which said magnet is not significantly oxidized, applying to said magnet at said elevated temperature an external demagnetizing field having a maximum value which is at least percent below the intrinsic coercive force of saidmagnet and cooling said magnet to room temperature.

4. A process according to claim 3 wherein R is samarium. 

2. A process according to claim 1 wherein R is samarium.
 3. A process for treating a cobalt-rare earth permanent magnet to demagnetize said magnet to decrease its open circuit induction Bo by at least 80 percent so that said open circuit induction Bo is substanTially or completely recoverable by remagnetization of said magnet, said magnet having a length to diameter ratio greater than one and an intrinsic coercive force in excess of 5, 000 oersteds and said magnet being a sintered product wherein the pores are substantially non-interconnecting and having a density ranging from about 87 to 100 percent and consisting essentially of compacted particulate alloy consisting essentially of a composition ranging from a single solid Co5R phase to a Co5R phase and a second solid Co-R phase in an amount up to about 30 percent by weight of said product and richer in rare earth metal content than said Co5R phase, where R is a rare earth metal or metals, which comprises bringing said magnet up to an elevated temperature ranging from 40 to 75 percent of its Curie temperature in an atmosphere in which said magnet is not significantly oxidized, applying to said magnet at said elevated temperature an external demagnetizing field having a maximum value which is at least 50 percent below the intrinsic coercive force of said magnet and cooling said magnet to room temperature.
 4. A process according to claim 3 wherein R is samarium. 